Hitherto, in shield tunnel boring machines for boring tunnels through soft and unstable ground in which a inner space of the shield body is partitioned by a bulkhead at a position behind a rotary cutter head on the forward end of the shield body for boring the tunnel face, it has been practically impossible to directly observe or accurately reliably realize the actual state of any accidental collapses of tunnel face ground in excess of the actual amount being excavated (which shall be referred to as "excess excavation" hereinafter). Such collapses frequently occur during the tunnel boring operation through soft and unstable ground. Specifically in the case of the hydraulic type boring machines utilizing generally muddy water as a liquid for hydraulic boring of the ground, such muddy water is completely opaque when excavated ground formations are mixed therewith so that direct observation of the tunnel face state can never be achieved even if an observing window is provided in the bulkhead. For this reason, there have been suggested certain measures of determining the occurrence of the excess excavation based on rapid change or increase in excavated ground formations which are drained out of the tunnel face together with the muddy water fed to the face. However, even with these measures, still it has been impossible to promptly determine the occurrence of the excess excavation since the measurements of the amount of drained ground formation involve an inherent time lag due to existing distance between the tunnel face and actual measuring position of the drained ground formation amount. Moreover, it is practically impossible to detect or measure the location, shape and the like of the excess excavation only by measuring the varying amount of the drained ground formations.